The results of a series of CD triaxial tests on a medium dense, cohesionless sand are summ...

The results of a series of CD triaxial tests on a medium dense, cohesionless sand are summarized in the table below. The void ratios for all the test specimens were approximately the same at the start of the test. Plot the strength circles and draw the Mohr failure envelope for this series of tests. What angle of internal friction should be used in solving stability problems in which the range of normal stresses is

(a) 0-500 kPa: (b) 1000-1500 kPa; (c) 3-6 MPa; and (d) 0- 6 MPa?